Neuroendocrinology of sexual plasticity in teleost fishes

Variation in gene transcript profiles of two V1a-type arginine vasotocin receptors among sexual phases of bluehead wrasse (Thalassoma bifasciatum)

The neurohypophyseal hormone arginine vasotocin (AVT) mediates behavioral and reproductive plasticity in vertebrates, and has been linked to the behavioral changes associated with protogyny in the bluehead wrasse (Thalassoma bifasciatum). In this study, we sequenced full-length cDNAs encoding two distinct V1a-type AVT receptors (v1a1 and v1a2) from the bluehead wrasse, and examined variation in brain and gonadal abundance of these receptor transcripts among sexual phases. End point RT-PCR revealed that v1a1 and v1a2 transcripts varied in tissue distribution, with v1a1 receptor mRNAs at greatest levels in the telencephalon, hypothalamus, optic tectum, cerebellum and testis, and v1a2 receptor transcripts most abundant in the hypothalamus, cerebellum and gills. In the brain, v1a1 and v1a2 mRNAs both localized by in situ hybridization to the dorsal and ventral telencephalon, the preoptic area of the hypothalamus, the ventral hypothalamus and lateral recess of the third ventricle. Quantitative real-time RT-PCR revealed that relative abundance of these two receptor mRNAs varied significantly in brain and gonad with sexual phase. Relative levels of v1a2 mRNAs were greater in whole brain and isolated hypothalamus of terminal phase (TP) male wrasse compared to initial phase (IP) males or females. In the gonad, v1a1 mRNAs were at levels 2.5-fold greater in the testes of IP males - and 4-5-fold greater in the testes of TP males - compared to the ovaries of females. These results provide evidence that V1a-type AVT receptor transcript abundance in the hypothalamus and gonads of bluehead wrasse varies in patterns linked to sexual phase, and bestow a foundation for future studies investigating how differential expression of v1a1 and v1a2 teleost AVT receptors links to behavioral status and gonadal function in fish more broadly.

Social plasticity in fish: integrating mechanisms and function

Social plasticity is a ubiquitous feature of animal behaviour. Animals must adjust the expression of their social behaviour to the nuances of daily social life and to the transitions between life-history stages, and the ability to do so affects their Darwinian fitness. Here, an integrative framework is proposed for understanding the proximate mechanisms and ultimate consequences of social plasticity. According to this framework, social plasticity is achieved by rewiring or by biochemically switching nodes of the neural network underlying social behaviour in response to perceived social information. Therefore, at the molecular level, it depends on the social regulation of gene expression, so that different brain genomic and epigenetic states correspond to different behavioural responses and the switches between states are orchestrated by signalling pathways that interface the social environment and the genotype. At the evolutionary scale, social plasticity can be seen as an adaptive trait that can be under positive selection when changes in the environment outpace the rate of genetic evolutionary change. In cases when social plasticity is too costly or incomplete, behavioural consistency can emerge by directional selection that recruits gene modules corresponding to favoured behavioural states in that environment. As a result of this integrative approach, how knowledge of the proximate mechanisms underlying social plasticity is crucial to understanding its costs, limits and evolutionary consequences is shown, thereby highlighting the fact that proximate mechanisms contribute to the dynamics of selection. The role of teleosts as a premier model to study social plasticity is also highlighted, given the diversity and plasticity that this group exhibits in terms of social behaviour. Finally, the proposed integrative framework to social plasticity also illustrates how reciprocal causation analysis of biological phenomena (i.e. considering the interaction between proximate factors and evolutionary explanations) can be a more useful approach than the traditional proximate-ultimate dichotomy, according to which evolutionary processes can be understood without knowledge on proximate causes, thereby black-boxing developmental and physiological mechanisms.

Female-specific target sites for both oestrogen and androgen in the teleost brain

To dissect the molecular and cellular basis of sexual differentiation of the teleost brain, which maintains marked sexual plasticity throughout life, we examined sex differences in neural expression of all subtypes of nuclear oestrogen and androgen receptors (ER and AR) in medaka. All receptors were differentially expressed between the sexes in specific nuclei in the forebrain. The most pronounced sex differences were found in several nuclei in the ventral telencephalic and preoptic areas, where ER and AR expression were prominent in females but almost completely absent in males, indicating that these nuclei represent female-specific target sites for both oestrogen and androgen in the brain. Subsequent analyses revealed that the female-specific expression of ER and AR is not under the direct control of sex-linked genes but is instead regulated positively by oestrogen and negatively by androgen in a transient and reversible manner. Taken together, the present study demonstrates that sex-specific target sites for both oestrogen and androgen occur in the brain as a result of the activational effects of gonadal steroids. The consequent sex-specific but reversible steroid sensitivity of the adult brain probably contributes substantially to the process of sexual differentiation and the persistent sexual plasticity of the teleost brain.

Diversity and plasticity of sex determination and differentiation in fishes

Among vertebrates, fishes show an exceptional range of reproductive strategies regarding the expression of their sexuality. Fish sexualities were categorized into gonochorism, synchronous/sequential hermaphrodite, or unisexual reproduction. In gonochoristic fishes, sex is determined genetically or by environmental factors. After sex determination, the gonads are differentiated into ovary or testis, with the sex remaining fixed for the entire life cycle. In contrast, some sequential hermaphrodite fishes can change their sex from male to female (protandrous), female to male (protogynous), or serially (bi-directional sex change) in their life cycle. In many cases, sex change is cued by social factors such as the disappearance of a male or female from a group. This unique diversity in fishes provides an ideal animal model to investigate sex determination and differentiation in vertebrates. This review first discusses genetic-orientated sex determination mechanisms. Then, we address the gonadal sex differentiation process in a gonochoristic fish, using an example of the Nile tilapia. Finally, we discuss various types of sex change that occur in hermaphrodite fishes.

Sexually dimorphic effects of melatonin on brain arginine vasotocin immunoreactivity in green treefrogs (Hyla cinerea)

Arginine vasotocin (AVT) and its mammalian homologue, arginine vasopressin (AVP), regulate a variety of social and reproductive behaviors, often with complex species-, sex- and context-dependent effects. Despite extensive evidence documenting seasonal variation in brain AVT/AVP, relatively few studies have investigated the environmental and/or hormonal factors mediating these seasonal changes. In the present study, we investigated whether the pineal hormone melatonin alters brain AVT immunoreactivity in green treefrogs (Hyla cinerea). Reproductively active male and female frogs were collected during the summer breeding season and a melatonin-filled or blank silastic capsule was surgically implanted subcutaneously. The duration of hormone treatment was 4 weeks, at which time frogs were eutha-nized and the brains and blood collected and processed for AVT immunohistochemistry and steroid hormone assay. We quantified AVT-immunoreactive (AVT-ir) cell bodies in the nucleus accumbens (NAcc), caudal striatum and amygda- la (AMG), anterior preoptic area, suprachiasmatic nucleus (SCN) and infundibular region of the ventral hypothalamus. Sex differences in AVT-ir cell number were observed in all brain regions except in the anterior preoptic area and ventral hypothalamus, with males having more AVT-ir cells than females in the NAcc, amygdala and SCN. Brain AVT was sensitive to melatonin signaling during the breeding season, and the effects of melatonin varied significantly with both region and sex. Treatment with melatonin decreased AVT immunoreactivity in both the NAcc and SCN in male H. cinerea. In contrast, brain AVT was relatively insensitive to melatonin signaling in females, indicating that the regulation of the AVT/AVP neuropeptide system by melatonin may be sexually dimorphic. Finally, melatonin did not significantly influence testosterone or estradiol concentrations of male or female frogs, respectively, suggesting that the effects of melatonin on AVT immunoreactivity are independent of changes in gonadal sex steroid hormones. Collectively, our results indicate that the AVT/AVP neuronal system may be an important target for melatonin in facilitating seasonal changes in reproductive physiology and social behavior.

Sex differences in the expression of vasotocin/isotocin, gonadotropin-releasing hormone, and tyrosine and tryptophan hydroxylase family genes in the medaka brain

In teleost fish, sex differences in several behavioral and physiological traits have been assumed to reflect underlying sex differences in the central expression of neurotransmitter/neuromodulator-related molecules, including vasotocin (VT)/isotocin (IT), gonadotropin-releasing hormone (GnRH), and tyrosine and tryptophan hydroxylases (TH and TPH). However, the sex-dependent expression patterns of these molecules have not been fully characterized in the teleost brain. In the present study, we therefore systematically evaluated sex differences in their expression in the medaka (Oryzias latipes) brain. The most prominent sex difference was observed in vt expression in the nucleus posterior tuberis (NPT) and the posterior part of the nucleus ventral tuberis (NVT) in the hypothalamus, where the expression was completely male-specific. Male-biased expression of gnrh1, tph1, and tph2 was also evident in the supracommissural and posterior nuclei of the ventral telencephalic area (Vs/Vp), medial nucleus of the dorsal telencephalic area (Dm), and thalamic dorsal posterior nucleus (DP), respectively. In contrast, the overall expression levels of it and gnrh3 were higher in the female brain than in the male brain. Equally importantly, no conspicuous sex differences were observed in the expression of gnrh2, th1, and th2, despite several previous reports of their sex-biased expression in the brains of other teleost species. Taken together, these data have uncovered previously unidentified sex differences in the expression of VT/IT, GnRH, and TPH in the teleost brain, which may possibly be relevant to sexual dimorphism in some behavioral and/or physiological traits, and have simultaneously highlighted potential species differences in the roles of these molecules.

Identifying context-specific gene profiles of social, reproductive, and mate preference behavior in a fish species with female mate choice

Sensory and social inputs interact with underlying gene suites to coordinate social behavior. Here we use a naturally complex system in sexual selection studies, the swordtail, to explore how genes associated with mate preference, receptivity, and social affiliation interact in the female brain under specific social conditions. We focused on 11 genes associated with mate preference in this species (neuroserpin, neuroligin-3, NMDA receptor, tPA, stathmin-2, β-1 adrenergic receptor) or with female sociosexual behaviors in other taxa (vasotocin, isotocin, brain aromatase, α-1 adrenergic receptor, tyrosine hydroxylase). We exposed females to four social conditions, including pairings of differing mate choice complexity (large males, large/small males, small males), and a social control (two females). Female mate preference differed significantly by context. Multiple discriminant analysis (MDA) of behaviors revealed a primary axis (explaining 50.2% between-group variance) highlighting differences between groups eliciting high preference behaviors (LL, LS) vs. other contexts, and a secondary axis capturing general measures distinguishing a non-favored group (SS) from other groups. Gene expression MDA revealed a major axis (68.4% between-group variance) that distinguished amongst differential male pairings and was driven by suites of “preference and receptivity genes”; whereas a second axis, distinguishing high affiliation groups (large males, females) from low (small males), was characterized by traditional affiliative-associated genes (isotocin, vasotocin). We found context-specific correlations between behavior and gene MDA, suggesting gene suites covary with behaviors in a socially relevant context. Distinct associations between “affiliative” and “preference” axes suggest mate preference may be mediated by distinct clusters from those of social affiliation. Our results highlight the need to incorporate natural complexity of mating systems into behavioral genomics.

Vasopressin, oxytocin, and social odor recognition

Central vasopressin and oxytocin, and their homologues, modulate a multitude of social behaviors in a variety of animal taxa. All social behavior requires some level of social (re)cognition, and these neuropeptides exert powerful effects on an animal's ability to recognize and appropriately respond to a conspecific. Social cognition for many mammals, including rodents, begins at the main and accessory olfactory systems. We recently identified vasopressin expressing neurons in the main and accessory olfactory bulb and in the anterior olfactory nucleus, a region of olfactory cortex that transmits and processes information in the main olfactory system. We review this and other work demonstrating that both vasopressin and oxytocin modulate conspecific social recognition at the level of the olfactory system. We also outline recent work on the somato-dendritic release of vasopressin and oxytocin, and propose a model by which the somato-dendritic priming of these neuropeptides in main olfactory regions may facilitate the formation of short-term social odor memories. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Nonapeptides and social behavior in fishes

The nonapeptide hormones arginine vasotocin and isotocin play important roles in mediating social behaviors in fishes. Studies in a diverse range of species demonstrate variation in vasotocin neuronal phenotypes across within and between sexes and species as well as effects of hormone administration on aggressive and sexual behaviors. However, patterns vary considerably across species and a general explanatory model for the role of vasotocin in teleost sociosexual behaviors has proven elusive. We review these findings, examine potential explanations for the lack of agreement across studies, and propose a model based on the parvocellular AVT neurons primarily mediating social approach and subordinance functions while the magnocellular and gigantocellular AVT neurons mediate courtship and aggressive behaviors. Isotocin neuronal phenotypes and effects on behavior are relatively unstudied, but research to date suggests this will be a fruitful line of inquiry. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Histological observation of the urogenital papillae in the bi-directional sex-changing gobiid fish, Trimma okinawae

The gobiid fish Trimma okinawae changes its sex bi-directionally according to its social status. Morphological changes in the urinogenital papillae (UGP) of this fish have been reported during sex change. However, there have been no detailed observations of such changes. Here, we histologically examined the UGP structure of male- and female-phase fish. UGPs of fish in female and male phase contained both oviducts and sperm ducts. Both ducts were coalesced into one duct within the posterior region of the UGP. Female-phase fish had many longitudinal folds in the hypertrophied tunica mucosa of the oviduct, which was found to be responsible for the transport of eggs and the removal of follicular cells from the oocyte. In contrast, male-phase fish had an immature oviduct and a mature sperm duct in the UGP. In the male-phase fish, the co-existence of spermatozoa and fibrillar secretions was observed in the sperm duct during spermiation.

Social Regulation of Gene Expression in the Hypothalamic-Pituitary-Gonadal Axis

Reproduction is a critically important event in every animals' life and in all vertebrates is controlled by the brain via the hypothalamic-pituitary-gonadal (HPG) axis. In many species, this axis, and hence reproductive fitness, can be profoundly influenced by the social environment. Here, we review how the reception of information in a social context causes genomic changes at each level of the HPG axis.

Steroid catabolism in marine and freshwater fish

Steroids play important roles in regulating many physiological functions in marine and freshwater fish. Levels of active steroid in blood and tissues are determined by the balance between synthetic and catabolic processes. This review examines what is known about pathways of catabolism of steroids, primarily sex steroids, in marine and freshwater fish. Cytochrome P450 (P450) isoforms present in hepatic microsomes catalyze steroid hydroxylation to metabolites with lower or no activity at estrogen or androgen receptors. Important pathways of steroid catabolism to readily excreted metabolites are glucuronidation and sulfonation of hydroxyl groups. Estradiol, testosterone, DHEA and hydroxylated metabolites of these and other steroids readily form glucuronide and sulfate conjugates in those fish species where these pathways have been examined. Little is known, however, of the structure and function of the UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) enzymes involved in steroid conjugation in fish. Glucuronide and sulfate conjugates of steroids may be transported into and out of cells by organic anion transporter proteins and multi-drug resistance proteins, and there is growing evidence that these proteins play important roles in steroid conjugate transport and elimination. Induction or inhibition of any of these pathways by environmental chemicals can result in alteration of the natural balance of steroid hormones and could lead to disruption of the endocrine system. Recent studies in this area are presented, with particular focus on phase II (conjugative) pathways.

Tight hormonal phenotypic integration ensures honesty of the electric signal of male and female Brachyhypopomus gauderio

Hormones mediate sexually selected traits including advertisement signals. Hormonal co-regulation links the signal to other hormonally-mediated traits such that the tighter the integration, the more reliable the signal is as a predictor of those other traits. Androgen administration increases the duration of the communication signal pulse in both sexes of the electric fish Brachyhypopomus gauderio. To determine whether the duration of the signal pulse could function as an honest indicator of androgen levels and other androgen-mediated traits, we measured the variation in sex steroids, signal pulse duration, and sexual development throughout the breeding season of B. gauderio in marshes in Uruguay. Although the sexes had different hormone titres and signal characteristics, in both sexes circulating levels of the androgens testosterone (T) and 11-ketotestosterone (11-KT) were strongly related to signal pulse duration. Consequently, signal pulse duration can serve as an honest indicator of circulating androgens in males and females alike. Additionally, through phenotypic integration, signal pulse duration also predicts other sexual traits directly related to androgen production: gonad size in males and estradiol (E2) levels in females. Our findings show that tight hormonal phenotypic integration between advertisement signal and other sex steroid-mediated traits renders the advertisement signal an honest indicator of a suite of reproductive traits.

Genes, hormones, and circuits: an integrative approach to study the evolution of social behavior

Tremendous progress has been made in our understanding of the ultimate and proximate mechanisms underlying social behavior, yet an integrative evolutionary analysis of its underpinnings has been difficult. In this review, we propose that modern genomic approaches can facilitate such studies by integrating four approaches to brain and behavior studies: (1) animals face many challenges and opportunities that are ecologically and socially equivalent across species; (2) they respond with species-specific, yet quantifiable and comparable approach and avoidance behaviors; (3) these behaviors in turn are regulated by gene modules and neurochemical codes; and (4) these behaviors are governed by brain circuits such as the mesolimbic reward system and the social behavior network. For each approach, we discuss genomic and other studies that have shed light on various aspects of social behavior and its underpinnings and suggest promising avenues for future research into the evolution of neuroethological systems.

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Sex differences in aromatase gene expression in the medaka brain

The brain of teleost fish exhibits a significant degree of sexual plasticity, even in adulthood. This unique feature is almost certainly attributable to a teleost-specific sexual differentiation process of the brain, which remains largely unknown. To dissect the molecular basis of sexual differentiation of the teleost brain, we searched for genes differentially expressed between both sexes in the medaka brain. One gene identified in the screen, cyp19a1b, which encodes the steroidogenic enzyme aromatase, was selected for further analysis. As opposed to the situation in most vertebrates, medaka cyp19a1b is expressed at higher levels in the adult female brain than the male brain. The female-biased expression in the brain is consistent regardless of reproductive or diurnal cycle. Medaka cyp19a1b is expressed throughout the ventricular zones in wide areas of the brain, where, in most regions, females have a greater degree of expression compared to males, with the optic tectum exhibiting the most conspicuous predominance in females. Contrary to what is known in mammals, cyp19a1b expression exhibits neither a transient elevation nor a sex difference in medaka embryos. It is not until just before the onset of puberty that cyp19a1b expression in the medaka brain is sexually differentiated. Finally, cyp19a1b expression in the medaka brain is not under the direct control of sex chromosome genes but relies mostly, if not solely, on oestrogen derived from the gonad. These unique properties of aromatase expression in the brain probably contribute substantially to the less rigid sexual differentiation process, thus ensuring remarkable sexual plasticity in the teleost brain.